Comparison and application of low-voltage fuses and circuit breakers

In the past ten years, in the planning of low-voltage power distribution systems for industrial buildings, civil buildings and outdoor equipment, the use of low-voltage fuses has become less and less, but thousands of people have used low-voltage circuit breakers. Correspondingly, low-voltage The number of fuses installed in the distribution box is also greatly reduced.

1 question

"How many years have passed, and still use the fuse!" "The fuse has expired!" This seems to make sense but has caused quite a lot of doubtful technical problems, which is placed in front of the electrical professional and technical personnel. For a long time, in the planning of low-voltage power distribution systems for industrial buildings, civil buildings and outdoor equipment, there are fewer and fewer low-voltage fuses. In the application of electrical distribution systems to protect electrical appliances, the author believes that this is an incorrect or incomplete knowledge and practice. Therefore, it is necessary to make some comparisons and analyses on fuses and circuit breakers (refer to low-voltage power distribution lines below) to kill readers in the future to be able to more correctly and reasonably replace these two kinds of protective appliances.

2 Protection of distribution lines and deployment of protective appliances

2.1 Distribution line protection requirements

In order to protect personal safety (electric shock caused by direct contact) in the event of defects (such as overload, short circuit and grounding defects), or to avoid damage to the circuit due to overheating, and even electrical fire, the circuit breaker circuit should have necessary protective measures to protect Because low-voltage power distribution lines are spread all over the interior and outdoors of various buildings, the probability of defects is high, and there may be a large number of non-artificial contacts, it is easier to take such protective measures.

These protections should conform to the relevant regulations of the national standard "Low-Voltage Power Distribution Standard" (GB50054-95). At all times, there is an improper setting of protective electrical appliances in the circuit, and it is necessary to correctly set its parameters to ensure that the circuit is reliably and selectively blocked at regular times, that is, the protective electrical appliance closest to the defect is required to act, and the superior protective electrical appliance does not act. , As the smallest scale of the intercepted circuit.

2.2 Types and deployment of protective appliances

There are two main types of protection appliances: one is a circuit breaker, and the other is a fuse. There are many types of circuit breakers. From the protection characteristics related to this article, there are two categories: non-selective and selective circuit breakers; in addition, there are circuit breakers with leakage protection. Each of these protective appliances has its own characteristics, and naturally has its shortcomings. They should be selected according to the specific conditions and requirements of the distribution system everywhere, and cannot be simply rated as "advanced" or "backward".

In today's world, especially in some developed countries, circuit breaker products and technologies have been developed very rapidly, and newer circuit breakers with better protection functions have been continuously developed. In recent decades, products have been updated every ten years or so, until the introduction of intelligent circuit breakers with perfect functions and communication modules, which provide excellent protection appliances for the protection of power distribution lines.

In the past two decades, the long-term electrical industry has expanded rapidly. Circuit breaker product manufacturers have closely followed the world's advanced technological trends and developed a variety of intelligent circuit breakers to provide more complete protection functions for switch circuits.

However, before some in Europe and America, the fuse was not screened because of the rapid deployment of circuit breakers, and the fuse was not regarded as "outdated" or "outdated" products. Well-known companies in Germany, France and other countries (such as these indicate that the circuit breaker is an advanced protective electrical appliance, not an "expired" or "outdated" product. It should be said that the two ends of the fuse and the circuit breaker complement each other and have their own uses.

3 Comparison of fuses and circuit breakers

Now compare the protective functions of fuses and circuit breakers and other features, during which the circuit breakers are described in two categories: non-selective type and selective type.

3.1 Fuse

3.1.1 The primary benefits of fuses

(1) Good selectivity: the fuse-link rated current of the upper and lower fuses immediately meets the requirements of the overcurrent selection ratio of the national standard GB13539.1-92, GB13539.2-92 and the IEC standard regulation of 1.6:1, that is, the upper level The rated current of the fuse link is not less than 1.6 times the value of the lower level, which is regarded as the upper level and the lower level can selectively stop the leakage and cut off the current;

(2) Good current limiting characteristics and high breaking ability;

(3) Relatively small scale;

(4) The price is relatively cheap.

3.1.2 The primary defect of the fuse

(1) It is necessary to replace the fuse link after the defect is blown;

(2) The protection function is single, once a period of over current inverse time characteristic, excessive, short circuit and grounding defects are all used this protection;

(3) When one-phase fuse occurs, the coaxial motor will cause undesirable consequences for two-phase operation. Of course, a fuse with an alarm signal can be used instead;

(4) The remote control cannot be completed. It is possible to combine with electric knife switch and load switch.

3.2 Non-selective circuit breaker

3.2.1 Primary interests

(1) After an absolute short-circuit, it can be reset manually without replacing the device, unless the large short-circuit current needs to be repaired;

(2) The two-stage protection function of the long delay release and the instantaneous overcurrent release with inverse time characteristics are used for repeated and short-circuit protection respectively, and each perform its own duties;

(3) The remote control can be completed when the organization is powered on.

3.2.2 Primary defects

(1) It is difficult to complete selective blocking and interception between upper and lower non-selective circuit breakers. The disadvantage is that when the current is short-circuited, it is very simple to cause uniform transition between upper and lower circuit breakers;

(2) The relative price is slightly higher;

(3) Some circuit breakers have a small amount of breaking capacity. For example, when a circuit breaker with a smaller rated current is installed close to a large-capacity transformer, the breaking capacity will be insufficient. Nowadays, products with high breaking ability can meet the requirements, but the price is higher.

3.3 Selective circuit breaker

3.3.1 Primary interests

(1) Stakeholders with the above-mentioned non-selective circuit breakers;

(2) It has a variety of protection functions, including long time delay, instantaneous, short time delay and grounding defect (including zero sequence current and residual current protection) protection, which can complete repeated, short-circuit, large short-circuit current transient and absolute grounding protection. , The protection sensitivity is extremely high, the dispatching of various parameters is convenient, and the simple and satisfactory switch circuit protection requirements. In addition, it can also have a cascade protection function, which has a better selective action function;

(3) At present, most of the selective circuit breaker products have intelligent features. In addition to protection functions, they also have power measurement, defect records, and communication interfaces to complete the centralized monitoring and processing of switching equipment and systems.

3.3.2 The first problem

(1) The price is very high, so it should only be used at the head end of the distribution line and on the branch trunk line of particularly important places;

(2) Scale competitiveness.

4 Distribution line characteristics and selection of protective electrical appliances

4.1 Characteristics of distribution lines and requirements for protection of electrical appliances

(1) Switching systems generally have trunk type and radial type, and there are other hybrid systems. Generally speaking, the trunk line of the trunk system has high requirements for the protection of electrical appliances, and often requires high-end protection of electrical appliances, so the selection type circuit breaker is generally used;

(2) Distribution lines can be divided into main lines, sub-main lines and end line types. The main line is the feeder wire drawn from the low-voltage power distribution panel of the substation. When it is a trunk line and the capacity of this main line is large, the main line is generally used;

(3) The end line is the line directly connected to the electrical equipment. In the case of a short circuit or a grounding short circuit, the circuit must be blocked quickly or instantaneously. There is no selective requirement.

4.2 Defect characteristics of distribution lines

(1) Short circuit and grounding defects occur in the length of the end loop, which accounts for more than 90% of all defects, especially socket loops are more accurate, because plugs, sockets and mobile electrical appliances and their wires and connectors are relatively simple to have shortcomings;

(2) In terms of the types of defects, the grounding defects account for about 80%~90%, and the short-circuit defects between phases are reduced;

(3) The ending circuit of the motor and other equipment is generally excessive, and the short circuit is prolonged. The overload of the motor accounts for more than 80% of the total number of defects. The overload is protected by a thermal relay and will not cause the fuse and circuit breaker to operate.

4.3 Protection electric appliance selection scheme

Based on the aforementioned circuit defect characteristics and the comparison of multiple protective electrical functions, suggestions for the selection of protective electrical appliances are proposed. Here only talk about the selection scheme of fuse and circuit breaker, and does not touch on the setting of protective electrical parameters.

4.3.1 Selective crusher should be selected for the following positions

(1) The main switch of the transformer's low-voltage outlet;

(2) The protection of the bus trunk line drawn from the low-voltage power distribution panel of the substation, or the trunk line with the drawn current capacity (such as 500A or more);

(3) Protection of radial lines of current capacity circuit breakers (such as 300A or more) drawn from low-voltage power distribution panels in important places.

4.3.2 Optional non-selective circuit breaker for the following positions

(1) End the protection of the loop;

(2) The protection of the upper-level sub-trunk line close to the end of the loop, when there are not many electrical equipment for supply, and the impact of accidental power failure is not too large.

4.3.3 Fuse should be selected for the following positions

(1) Protection of expected sub-trunks in the middle of distribution lines;

(2) Protection of the main line with small current capacity (such as less than 300A) drawn from the low-voltage distribution panel of the substation;

(3) The protection of the motor termination circuit can also be used when conditions permit. However, it is not suitable to replace the gG type fuse (ie full-scale breaking, general purpose fuse), but should choose aM type fuse (ie partial proportional breaking, Fuse for motor protection). Because the rated current of the fuse-link selected by the aM type fuse is smaller than that of the gG type, it is conducive to the sensitivity of the forward protection, and it also avoids the selection of the upper-level protection device.